1. Field of the Invention
This invention relates to communication systems and particularly to communication systems providing polarity reversal to a transmission line.
2. Description of the Related Art
Communication systems utilizing transmission lines such as subscriber loops are commonplace throughout much of the world. Subscriber loops are terminated on one end by terminal equipment and provide a path for the terminal equipment to communicate with other terminal equipment via, for example, a vast network of central offices, private branch exchanges, satellite relay systems, transmission lines, repeaters, and wireless systems. FIG. 1 illustrates a portion of a conventional subscriber loop communication system 100. The subscriber loop 102 is modeled as a balanced two-wire transmission line 103, with loop resistances R and inductances L and leakage impedance modeled by capacitor 104 and resistor 106. The subscriber loop 102 is terminated on respective ends by terminal equipment 110 and a central office 122 line card 120. The subscriber loop 102 provides a communication path for information transmission such as voice signals and signaling information between a subscriber's terminal equipment and the central office 108.
Terminal equipment 110 is illustratively modeled as a telephone with off-hook resistance 112, nominally 200 ohms, and ringer impedance Z, which may be modeled, for example, as a series RC or series RLC circuit. Terminal equipment 110 includes a switch hook 118 which loads the subscriber loop 102 on the subscriber end with resistance 112 when the terminal equipment 110 is off-hook (as shown) and loads subscriber loop 102 with ringer impedance Z when the terminal equipment 110 is on-hook. Terminal equipment 110 may be any of a variety of devices besides the familiar, ubiquitous telephone such as facsimile machines, private branch exchanges, voice mail systems key telephone systems, computers, modems, telephone answering machines, alarm systems, and radio control systems, as well as many other devices.
The other end of the subscriber loop 102, opposite terminal equipment 110, converges on line card 120 of central office 122. The line card 120 terminates subscriber loop 102 at conductors A (Tip) and B (Ring) with a feed impedance of 900 ohm or other standard feed impedance. The line card 120 provides a gateway to the public switched telephone network (PSTN) through switching network 124.
Referring to FIG. 2, the subscriber line interface circuit (SLIC) 202 of line card 120 provides a two-wire interface 204 to the generally analog signal carrying subscriber loop 102. The SLIC 202 performs a variety of interface functions that allow terminal equipment 110 to communicate with other terminal equipment (not shown). The SLIC 202 and the subscriber loop audio-processing circuit (SLAC) 206 carry out the well-known BORSCHT (Battery feed, Overvoltage protection, Ringing, Supervision, Coding, Hybrid, and Test) functions. The SLIC 202 monitors direct current (DC) levels on the subscriber loop 102 with ground key detector circuitry 208 and off-hook detector circuitry 210. Input decoder and control circuitry 214 provides a mechanism for other circuitry (not shown) in the central office 122 and for SLAC 206 to control such SLIC 202 functions as subscriber loop 102 activation, ringing, and polarity reversal. Analog two-wire interface 204 and signal transmission circuitry 212 cooperate in sensing subscriber loop 102 metallic voltage (voltage at conductor A minus voltage at conductor B or Vab) while generally having a high rejection of longitudinal voltages (Vab.sub.-- long). Alternating current (AC) signals, such as voice signals, are transmitted over subscriber loop 102 to terminal equipment 110 by two-wire interface 204 and signal transmission circuitry 212 in response to voice information input signals received from central office 122 through SLAC 206. The ring relay driver 216 activates a relay(s) which connects a ringing signal from a central office 122 ringing AC voltage generator (not shown) and DC voltage bias source, to terminal equipment 110 when a third party is calling. While terminal equipment 110 is on-hook, switch hook 118 connects subscriber loop 102 across the ringer impedance Z (FIG. 1). The ring trip detector circuitry 218 detects an off-hook condition of terminal equipment 110 and initiates cessation of the ringing voltage signal application to subscriber loop 102.
The SLAC 206 generally filters and converts analog output signals received from SLIC 202 into digital signals (A/D), processes the signals in accordance with control and timing information, and compresses the digital signals. The pulse code modulation (PCM) interface 220 provides PCM signals to the central office 122. SLAC 206 also generally receives digital audio input signals from the central office 122 via PCM interface 220, expands the digital input signals, processes the signal in accordance with control and timing information, and converts the digital signals into analog signals (D/A) for input to SLIC 202. Additional information on SLICs and SLACs is found in the 1995 Advanced Micro Devices of California data book entitled "Linecard Products for the Public Infrastructure Market."
The power feed controller 222 includes a battery feed circuit and a polarity reversal circuit. The battery feed functions supply direct current from a central office battery (not shown) to the subscriber loop 102 through balanced feed resistances at conductors A and B. Loop current is generally limited to no more than 45 to 75 milliamperes (mA) in a low-resistance subscriber loop. Higher subscriber loop resistances generally result in lower subscriber loop current. The on-hook subscriber loop powering voltage is typically the battery voltages minus 48 Volts DC (Vdc) less any overhead voltage, typically about 4 Vdc, necessary to prevent SLIC 202 saturation. Battery feed specifications are regionally provided and conform with specifications provided by, for example, BELLCORE, the Electronic Industries Association (EIA), British Telecom, and the International Telegraph and Telephone Consultative Committee (CCITT).
Public terminal equipment, such as public pay phones and hotel/motel phones, are commonly provided on a fee for use basis. Polarity reversal of subscriber A and B conductors is commonly used to initiate charges. Periodic polarity reversals during an on-going communication may be used to provide an indication to the user of elapsed connection time and indicate incurred charges. Polarity reversals may also provide a control signal to the terminal equipment for performing any of a number of well-known tasks.
For example, assuming that terminal equipment 110 is a public pay phone, when a user lifts the handset from the cradle, the phone goes off-hook, and a low resistance 112 is connected across the subscriber 102 loop A and B conductors by switch hook 118. Line card 120 responds to the current change by providing a dial tone. Normal subscriber loop polarity is conductor A grounded and conductor B at a nominal -48 V. The user dials an identification number of another terminal equipment, and a connection is established. When the called party answers and answer supervision is passed through the switching network 124, line circuit 120 immediately reverses battery polarity, thus, switching conductor A to -48 V and conductor B to ground. Regional specifications generally dictate the polarity reversal transition time to be no more than 64 milliseconds (ms).
During polarity reversal, audible frequency bias current changes are undesirable as they may cause noise to be transmitted through the phone transmitter to the ear of the user. The problem is especially acute when the impedance of subscriber loop 102 is large because small voltage changes cause large magnitude current changes. Thus, loud audible noises may be generated.
FIG. 2b illustrates a conventional approach to achieving polarity reversal. An increasing stair step voltage waveform 224 is generated by power feed controller 222 to change the voltage of conductor B from -48 V to +48 V. The waveform 224 is applied to the input terminals of a low pass filter 226 to polish the high frequency components of waveform 224. However, the low pass filter 226 output waveform 228 may still generate audible frequency current changes due to imperfect filtering and variable subscriber loop 102 impedance conditions. Likewise, the low pass filter 226 also produces a decreasing output waveform 232 from the decreasing stair step voltage input waveform 230 that may likewise generate audible frequency current changes in subscriber loop 102.
Thus, obtaining silent polarity reversal, especially in a dynamic environment, with conventional analog circuitry is a non-trivial problem. Furthermore, even if analog circuitry functions adequately for a given subscriber loop impedance, it may not function for the entire range of static and dynamic subscriber loop impedances for a given communication system. Moreover, the analog circuitry may not function reliably over time as analog circuitry often suffers from well-known aging side effects such as instability and circuit parameter drifting which may affect long-term reliability. Moreover, analog circuit features in an integrated circuit are large, presently in general on the order of about 7 .mu.m, which increases costs of analog integrated circuitry. Furthermore, providing a cost efficient common hardware platform which lends itself to cost effective modifications to conform with various regional standards and operating environments is at least a difficult problem.